PCBN cutting tools-2

Application of cubic boron nitride tools in modern industry

(HY-industry technical centre)

With the rapid development of the machinery manufacturing industry, more and more difficult-to-machine materials are widely used, and consequently, machining tools are also facing severe tests. Research on good wear resistance, strong thermal stability, and excellent impact resistance Superhard cutting tools are an inevitable trend. Efficient machining, hard machining, dry machining, ultra-precision machining and the processing of new difficult-to-machine materials are the development direction of the tool industry.

1 cubic boron nitride cutter

1.1 Preparation method of PCBN:
At present, the preparation of PCBN is mainly divided into the following three categories: there are bonding agents, pure PCBN and surface plated PCBN.

1.1.1 With PCBN:
According to the different binders contained in PCBN, they are divided into metal binders composed of metals and their alloys. Such binders play a good role in improving toughness, but the binder softens under high temperature conditions and has a side effect on wear resistance; Pure ceramic bond type, although it can solve the problem of softening at high temperature, but this kind of impact resistance is poor, and the service life is short; the bond of ceramic and metal or metal alloy, the bond of mixture of ceramic and metal, then Make up for the respective shortcomings of pure metal bond and pure ceramic bond. Companies in advanced countries in the world, such as American companies, British companies, and Sumitomo Corporation of Japan, have invested a lot of manpower and material resources in the research and application of metal ceramic composite polycrystalline cubic boron nitride, and have conducted continuous experimental research on it. Commercialization.

1.1.2 Pure PCBN:
Due to the presence of non-CBN components in the PCBN with a binder, the hardness and strength of the CBN are reduced, which directly leads to a reduction in its wear resistance, so pure PCBN came into being. Sumitomo Corporation of Japan developed almost pure CBN in 2000, and its CBN content is greater than 99.9%. Because it uses hexagonal boron nitride (hBN) with a catalyst and is directly converted to CBN at high temperature and pressure, its hardness and thermal stability are much greater than ordinary PCBN. Although there are almost no other compounds in the pure system, which show properties and characteristics close to single crystals, but its preparation conditions are demanding. At present, only a few companies can synthesize, and more research is needed from large-scale commercialization. The other is the use of hexagonal boron nitride directly converted into nano-polycrystalline cubic boron nitride through ultra-high temperature and high pressure. Due to its finer particles, and the particles are isotropic, there is no dissociation surface. But the preparation conditions are more severe. Usually it needs to be at a high temperature of 2000 ° C and a pressure above 7.7GPa. Due to the strict requirements on the sintering conditions, it is not suitable for industrial production. Therefore, the use of suitable adhesives to bond CBN is still a widely used method for preparing PCBN.

1.1.3 Surface plating PCBN:

Surface plating mainly refers to the surface of PCBN, by physical vapor deposition method, the surface is coated with a layer of ceramic coating of about 1-3μm, such as nitride. In addition to the cutting surface, the tool usually provides a “grindstone” on the cutting edge. For high negative chamfer angles, a large knife edge grindstone will generate greater stress and more heat. According to reports, a large amount of residual stress is generated on the processed surface. The solution to this problem is to extend the fatigue life through the surface coating. There is ample evidence that the coating can improve the performance of the cutting tool. The surface plating shows different tribochemical wear during application. When the processed steel contains different sizes of quenched carbon, the coating is easy to wear, which has an adverse effect on the performance of the cutting tool, so it is difficult to determine the thickness range of the coating.

1.2 The structure of CBN tool:

  • Integral CBN cutters: Integral CBN cutters have good wear resistance and impact resistance, especially for processing cast iron materials containing less than 10% ferrite. It can be roughed or finished with a large margin, which greatly improves the processing efficiency and reduces the processing cost. As more and more high-hardness castings are produced, this tool will be favored by more customers. There will be a great breakthrough in market share in the future. PCBN monolithic blades have developed rapidly in China in recent years, and have surpassed Europe, the United States, Japan and other developed countries. They have made important contributions in heavy-duty machining industries such as rollers, slurry pumps, and Raymond machines. Compared with traditional tools, the life of a single blade is increased by 4-6 times, the surface roughness reaches Ra0.2-0.4, and the grinding is replaced by turning; the processing efficiency can reach 3-5 times of the carbide tool.

  • Compound CBN tool: The compound CBN tool is a better choice of cemented carbide matrix. The CBN material is welded to the blade edge of the blade. Its wear resistance is better. It is mainly used for small hardness and more than HRC45. Finishing process can get good finish and dimensional accuracy.

  • Inlay type CBN tool: The so-called inlay type tool has a complete top surface, sintered on a cemented carbide substrate, and only provides multiple cutting edges on one side of the tool. This tool is cheaper than the compound tool. The multi-edge cutting greatly reduces the processing cost, but it is mainly used in the finishing process. The depth of the single-sided knife is controlled within 0.3mm, which can achieve high-speed turning, and the finish can reach Ra0.8-0.4 .

1.3 Application of PCBN

  • PCBN tools are mainly used for processing hard materials above HRC45, such as various hardened steels (carbon tool steel, alloy steel, bearing steel, die steel, high-speed steel, etc.), cast iron (vanadium titanium cast iron, high phosphorus cast iron, cold Hard cast iron, Ni-hard cast iron, etc.), high-temperature alloys (nickel-based alloys Inconel, Monel, Incoloy, Waspaloy, etc., cobalt-based alloys Stellite, Colmonoy, etc.), cemented carbide, sintered iron, surface thermal coating (welding) materials, etc. High-hardness and wear-resistant materials, powder metallurgy products, high cobalt hard alloys, etc., can also be used for processing titanium alloys, pure nickel, pure tungsten and other materials.

  • It can be predicted that the development of high-speed cutting will be more rapid in the future. When the era of ultra-high-speed cutting above 1000m / min comes, it will be the most powerful tool material and has great advantages. In the next 5-10 years, domestic tool material research will develop in the direction of fine process research, improvement of stability and strength, development of hardened steel composite sheets and expansion of sheet diameter; domestic PCBN tool materials must eventually enter the high-end market and be cast , Powder metallurgy, high temperature alloy, quenched steel, aviation stainless steel and other materials processing ushered in a broad prospect. The application of nano-powders and sub-nano-powders, the refinement and homogenization of powders is a development trend of cutting tools.

2 cubic boron nitride abrasive

2 1 Characteristics of CBN wheels with different binding agents

  • Metal bond CBN grinding wheel has the advantages of high strength, good toughness, good heat resistance and long service life. But its biggest disadvantage is its poor self-sharpness and high grinding heat. The research found that adding the second phase of ceramic, resin and graphite to the metal bond can significantly improve the defects such as poor self-sharpness of the metal bond CBN grinding wheel. The performance of the binder has been greatly improved.

  • Electroplated and brazed CBN grinding wheels have an exposed height of abrasive particles above 40%, which greatly increases the chip space and enables the abrasive particles to obtain a more ideal sharp shape, so a single-layer metal bond CBN grinding wheel can be applied For high-speed and efficient grinding. The ordering of grinding wheels developed in recent years has resulted in a regular and ordered arrangement of abrasive particles on the surface of the grinding wheel. Its advantage is that the effective abrasive particle ratio can increase the chip space of the grinding wheel, reduce the grinding heat, and improve the processing workpiece. Quality, the life of super-hard grinding wheels using ordered arrangement of abrasive particles is several times that of random arrangement.

  • The resin bond CBN grinding wheel has the advantages of good self-sharpening, not easy to block, certain elasticity, less grinding heat, low grinding force and high surface quality of the processed workpiece. However, the resin binders currently used in the super-hard material industry mainly include polyimide and phenolic resin, which generally have poor heat resistance, and are easily softened or decomposed during the grinding process, resulting in a decrease in adhesion and a large amount of abrasive particles falling off. . Therefore, in recent years, researchers have endeavored to develop resin binders with high heat and wear resistance, good toughness, and high mechanical strength. The methods of improving the heat resistance and toughness of the resin binder are contradictory, and it is difficult to satisfy both. Therefore, it is an important research direction for resin binders to promote the resin binder to satisfy the heat resistance and toughness at the same time by different methods.

  • Ceramic bond CBN grinding wheel, because it has the advantages of better combination with CBN abrasive particles, and the grinding wheel has a high grinding rate, good self-sharpness, high heat resistance, easy dressing, and long service life , And can maintain the integrity of the workpiece surface, so rapid development in recent years.

Compared with other bond CBN, ceramic bond CBN grinding wheel has the following characteristics:

  • Controllable porosity is beneficial to chip removal, so the grinding temperature is lower, reducing or avoiding grinding burns;

  • The grinding force of the grinding wheel is small, lower than the grinding energy, and the grinding performance is good;

  • Good heat resistance makes CBN abrasives fully utilized and the service life of grinding wheels is long;

  • The coefficient of thermal expansion is small, the rigidity is good, and the knife is small during grinding, which is suitable for high precision and low roughness grinding;

  • Easy to repair, can be completed at one time, and the maintenance cost is low;

  • The self-sharpening is better, the trimming interval is longer, and the trimming frequency is reduced;

  • Good chemical stability, wide range of application to grinding fluid;

  • Surface integrity of the grinding workpiece is good, the quality of the workpiece is high, and the service life is long.

2.2 Application of cubic boron nitride

Cubic boron nitride (CBN) grinding wheels are widely used in high-tech manufacturing. This article only selects a few examples to prove:

2.2.1 Application in grinding of marine crankshaft

The ship crankshaft is a key component of a ship’s engine and is regarded as the “heart” of the ship’s diesel engine. It plays a vital role in the safety of the ship. Its processing quality and processing efficiency directly affect the quality and cost of the engine. In order to improve the quality and processing efficiency of crankshafts, the use of high-speed CBN grinding wheels has become one of the general trends. From the perspective of medium and long-term scientific and technological development, the Ministry of Industry and Information Technology has supported two major national science and technology projects related to high-speed grinding of crankshafts.

2.2.2 Application in the efficient and precise machining of aero engine parts

  • In recent years, the aviation industry has continuously increased the requirements for engine performance, the structure of parts has become more and more complicated, the performance of the materials used has also been continuously improved, and the processing difficulty has become more and more difficult. The application of high-speed, high-efficiency and high-precision CBN grinding wheels in the grinding of aeroengine parts can greatly improve the machining efficiency and the quality of the parts, and it has become one of the research directions in the field of efficient precision machining.

  • As an important tool in high-efficiency precision grinding, CBN grinding wheels have the characteristics of good thermal conductivity, good stability and high durability, and are particularly suitable for the processing of difficult-to-machine materials such as titanium alloys and nickel-based superalloys. Therefore, more and more experts and scholars pay attention to the application of CBN grinding wheels in the efficient and precise grinding of aviation parts.

2.2.3 Grinding the inner circle of the bearing

  • Bearings are known as the “heart” parts of equipment manufacturing. The grinding wheel is a key tool for bearing grinding, which determines not only the bearing accuracy and surface quality, but also the energy saving, environmental protection and efficiency of the grinding process. Compared with ordinary grinding wheels, the machining efficiency of CBN grinding wheels can be nearly doubled, the service life is increased by about 100 times, the dressing interval time is extended by nearly 200 times, and the waste residue is reduced by about 90%.

2.2.4 Application of high-speed CBN grinding wheels in machining

Ceramic CBN grinding wheel, raw material production process, grinding wheel manufacturing process and use process of grinding wheel are extremely low in energy and resource consumption. They are energy-saving high-tech products that are unmatched by other tools.

(1) To save raw materials and energy, take the processing of camshaft and crankshaft as an example for analysis:

Grinding camshaft processing, from the actual production process, the ordinary grinding wheel is frequently replaced, and the number of dressing times

More, the use of high-speed ceramic CBN grinding wheels can replace corundum grinding wheels to increase productivity by more than 30%.

Grinding crankshaft processing, high-speed CBN grinding wheel has high durability, and the dressing frequency is very small, while the corundum grinding wheel dressing frequency is about 100 times that of high-speed CBN grinding wheel. Therefore, the use of high-speed CBN grinding wheels can greatly reduce the number of grinding wheel dressings, which greatly reduces the dust pollution generated during the grinding wheel dressing.

(2) Reduce processing procedures, shorten production cycle, and reduce production costs. Similarly, take the processing of camshaft and crankshaft as an example for analysis:

With camshaft grinding, the higher production efficiency of CBN grinding wheels reduces labor costs and enterprise management costs, and the overall production cost is reduced by 60% compared to ordinary corundum grinding wheels.

Crankshaft grinding adopts ceramic CBN grinding wheel, which not only saves rough machining processes such as turning and turning dumplings, but also shortens the heat treatment time required between roughing and finishing, which greatly shortens the production cycle.

3. Conclusion

The wide application of super-hard tools will further promote the development of high-tech manufacturing, and will also bring unprecedented opportunities and challenges to super-hard tool material producers and super-hard tool manufacturers. As a high-speed CBN grinding wheel, it is also an effective means to achieve green processing goals (less processing time, high production efficiency, low environmental pollution, and low comprehensive cost), and it is a weapon to achieve green processing and promote the development of green manufacturing. With the rapid development of China’s aerospace and automobile manufacturing industries, the status of cubic boron nitride as a processing tool will be irreplaceable and will be more widely used.

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